High Intraspecific Competition Research Articles

Morphometric and genetic divergence in island and mainland populations of Anolis nebulosus (Squamata: Polychrotidae) from Jalisco (Mexico): an instance of insular gigantism

The clouded anole Anolis nebulosus (Squamata: Polychrotidae) is widespread on the Pacific coast of Mexico. The species also inhabits Don Panchito, a small islet located near the coast of the Chamela-Cuixmala Biosphere Reserve in the state of Jalisco. We studied the extent of intraspecific differences in morphology (absolute size and body proportions) and in mtDNA sequences (16S and NDH2) between the population living on the islet (N = 18 for morphometry; N = 12 for mtDNA) and the one on the facing mainland (N = 38 for morphometry; N = 16 for mtDNA). The individuals on the islet are larger than those on the mainland with little overlap in size for either males (islet: 52.79 ± 1.82 mm; mainland: 40.96 ± 2.99 mm) or females (islet: 46.18 ± 3.24 mm; mainland 37.14 ± 2.13 mm). The presence of insular gigantism, as here found in A. nebulosus, seems uncommon in the genus and could be explained as a combination of low predation pressure and higher intraspecific competition on the island. Moreover, we found that sexual dimorphism (SD) is higher in the island population than in the mainland one. The molecular analysis shows the absence of shared haplotypes between the island and mainland populations. Ten mtDNA haplotypes belonged to the mainland population and three to the island population. The shape of the minimum spanning network and of the mismatch distribution indicates a single colonization event. These molecular data indicate a certain degree of isolation of the island population notwithstanding its proximity to the coast. The morphological characteristics of the anoles on Don Panchito match with the expectation of the so-called “reversed island syndrome” theory, which predicts an increased body size and sexual dimorphism in lizards living on very small islands characterized by unpredictable environmental conditions.

Trophic characteristics of a mangrove fish community in Southwest Thailand: Important mangrove contribution and intraspecies feeding variability

Mangrove production has been found to make a major contribution to the nutrition of a fish community in the Sikao Creek mangrove estuary, Southwest Thailand. Gut content analysis and carbon and nitrogen stable isotope analysis were used to assess fish feeding behavior and trophic reliance on different primary producers (mangrove leaves, phytoplankton, microphytobenthos) focusing on 19 dominant fish species, and 4 potential fish food items. Cluster analysis identified 5 trophic groups and the IsoSource model indicated the importance of primary food sources in trophically supporting different fish species. Most analyzed fish species had carbon isotopic signatures that were more depleted than those reported in previous studies, and the IsoSource model indicated that mangrove leaves were an important primary food source. This may be a specific characteristic of our study site, which is not well connected to other productive coastal habitats that provide alternative primary food sources. Thus we suggest that food chains in trophically isolated mangrove estuaries of southwest Thailand are more dependent on mangrove tree production. We also assessed the relationship of individuality in fish feeding habits and variability of δ13C values and showed that several mangrove fish species have significant intraspecies variability in feeding habits, possibly due to high intraspecific competition.

Wing Shape as an Indicator of Larval Rearing Conditions forAedes albopictusandAedes aegypti(Diptera: Culicidae)

Estimating a mosquito's vector competence, or likelihood of transmitting disease, if it takes an infectious bloodmeal, is an important aspect of predicting when and where outbreaks of infectious diseases will occur. Vector competence can be affected by rearing temperature and inter- and intraspecific competition experienced by the individual mosquito during its larval development. This research investigates whether a new morphological indicator of larval rearing conditions, wing shape, can be used to distinguish reliably temperature and competitive conditions experienced during larval stages. Aedes albopictus (Skuse) and Aedes aegypti (L.) (Diptera: Culicidae) larvae were reared in low intraspecific, high intraspecific, or high interspecific competition treatments at either 22 or 32 degrees C. The right wing of each dried female was removed and photographed. Nineteen landmarks and 20 semilandmarks were digitized on each wing. Shape variables were calculated using geometric morphometric software. Canonical variate analysis, randomization multivariate analysis of variance, and visualization of landmark movement using deformation grids provided evidence that although semilandmark position was significantly affected by larval competition and temperature for both species, the differences in position did not translate into differences in wing shape, as shown in deformation grids. Two classification procedures yielded success rates of 26 - 49%. Accounting for wing size produced no increase in classification success. There seemed to be a significant relationship between shape and size. These results, particularly the low success rate of classification based on wing shape, show that shape is unlikely to be a reliable indicator of larval rearing competition and temperature conditions for Ae. albopictus and Ae. aegypti.

Wildfire provides refuge from local extinction but is an unlikely driver of outbreaks by mountain pine beetle

Bark beetle outbreaks and wildfire are important disturbances in conifer ecosystems, yet their interactions are not well understood. We evaluated whether fire injury increased susceptibility of lodgepole pines (Pinus contorta) to mountain pine beetle (Dendroctonus ponderosae Hopkins), how it influenced beetle reproductive success, and whether beetle population phase altered this interaction. Eight sites that experienced wildfire and eight unburned sites were examined in the Greater Yellowstone Ecosystem (USA). Half were in areas where D. ponderosae was undergoing outbreaks, and half were in areas with low populations. We examined 2056 trees one year after fire for burn injury and beetle attack. We quantified beetle reproductive success in a random sample of 106 trees, and measured gallery areas of D. ponderosae and competing subcortical herbivores in 79 additional trees. Baited flight traps sampled stand‐level populations of subcortical herbivores and predators.Wildfire predisposed trees to D. ponderosae attack, but nonlinearly, with moderately injured trees being most preferred. This tree‐level interaction was influenced by stand‐level beetle population size, in that both healthy and fire‐injured trees of all classes were attacked where populations were high, but no healthy trees, and only low and moderately injured trees were killed where populations were low. The number of adult brood produced per female was likewise curvilinear, being highest in moderately injured trees. This reflected an apparent trade‐off, with high intraspecific competition arising from the large number of beetles needed to overcome defenses in healthy trees, vs. high interspecific competition and low substrate quality in more injured trees.These results suggest that fire‐injured trees can provide a reservoir for D. ponderosae during periods when populations are too low to overcome defenses of healthy trees, and might otherwise face localized extinction. However, the likelihood of populations increasing from endemic to outbreak levels in response to increased susceptibility is offset by the opposing constraints of lower substrate quality and higher competitor load in severely injured hosts, and the relative scarcity of moderately injured trees. Wildfire may confer some reproductive increases to populations already outbreaking. We present a conceptual model of how these disturbances and inherent feedbacks interact to affect beetle population dynamics.

Fluctuating asymmetry and Echinospartum horridum fitness components

In the last two decades, fluctuating asymmetry (FA) has been used commonly as an indicator of the stress experienced by individual organisms during development, although the technique has not been without controversy. In general, estimates of FA in populations have been positively correlated with measures of stress but, in the few studies in which it has been investigated, the correlations between estimates of FA and stress in individuals have been inconsistent. This study compared the FA in the opposite spines of stems and flowers to other bio-indicators of physiological stress in plants, such as annual crown growth ratio, annual flower production, annual crown death ratio, and branch fractal dimension. The position of an individual Echinospartum horridum within a patch influenced the stress conditions, and influenced its colonization strategy. Plants in the interior of patches are under high intraspecific competition due to negative density feedback (low flower production and branch biomass, and high annual crown death ratio), showed larger flower FA and spines FA repeatability (within-individual variation in FA) than plants at the periphery. Plant growth rates and the FA of spines were negatively correlated, but flower asymmetry and flower production was not significantly correlated, which reflects the capacity of E. horridum to adjust flower production to its developing conditions. We concluded that because organisms respond differently to different conditions, enhanced by the plasticity of plants in the development of morphological structures in response to changes in the environment, it is advisable to use multiple parameters to assess physiological stress in plants.

Inter-plant variation of grain yield components and kernel composition of maize crops grown under contrasting nitrogen supply

High intra-specific competition pressure, which is common at most maize ( Zea mays L.) cropping conditions, promotes inter-plant variation and the appearance of extreme plant hierarchies with different ability to capture scarce resources (i.e., dominant and dominated plants) within a stand. The objectives of the current work were to analyze (i) inter-plant variation of grain yield per plant (GYP), GYP components (KNP: kernel number per plant; KW: kernel weight), and kernel composition, together with those of their physiological determinants, i.e., plant growth (PG) rate around silking (PGR S), PGR S per kernel (PGR S KNP −1) and PG during the effective grain-filling period per kernel (PG GF KNP −1), under contrasting N supply and (ii) the contribution of dominant and dominated plants to changes in inter-plant variation and mean values of the studied traits. For these purposes two maize hybrids previously characterized by their contrasting inter-plant variation under N stress (low: AX820 and high: AX877) were cultivated at high stand densities (9 and 12 pl m −2) at two N supplies (N 0: control and N 200: 200 kg N ha −1) without water restrictions. For AX820, PGR S data set at both N levels explored a similar range (1–7.4 and 1.2–7.4 g pl −1 d −1 for N 0 and N 200, respectively) with a positive skewness in N 0, and an almost normal distribution of data in N 200. In contrast, for AX877, inter-plant variation of PGR S exhibited a normal distribution in both N levels, and N fertilization only produced a displacement of data to higher PGR S values (0–4.3 and 0.7–5.7 g pl −1 d −1 for N 0 and N 200, respectively). The effect of inter-plant variation of PGR S on the coefficient of variation (CV) of KNP was of a greater magnitude in AX877 than in AX820 due to the more linear KNP response to PGR S of the former. For both hybrids, mean values of KW increased and the CVs decreased in response to high N supply. Differences among plants and N levels in KW were related to the duration of the effective grain-filling period. Inter-plant variation of protein and starch concentrations was higher in N 0 than in N 200, but that of oil concentration was not affected by N supply. The analysis of plant hierarchies resulted useful to understand changes in mean values and frequency distributions of several agronomic traits.

Does Wildfire Increase the Risk of Moutain Pine Beetle Outbreaks in Lodgepole Pine Forests of the Greater Yellowstone Ecosystem?

We examined whether wildfire injury increased lodgepole pine, Pinus contorta, susceptibility to mountain pine beetle, Dendroctonus ponderosae, how it affects beetle reproduction, whether this interaction differs between endemic and epidemic populations, and how wildfire influences tree defense physiology. Wildfire predisposed trees to mountain pine beetle attack. In particular, fire-injured trees had a lower ability in synthesized monoterpenes in response to simulated attacks than did non-injured trees. However, beetles responded in a non-linear fashion; moderately-injured trees were most preferred. This interaction was influenced by beetle population size. Healthy and fire-injured trees were attacked when populations were high, but no healthy trees and no severely-injured trees were killed when populations were low. Beetle brood production per female was also curvilinear being highest in moderately-injured trees. This reflected a trade-off between high intraspecific competition arising from the large number of beetles needed to overcome defenses in healthy trees, and high interspecific competition and low substrate quality in severely injured trees. These results suggest that fire-injured trees can provide a resource for mountain pine beetles during the extended periods when populations are not high enough to overcome defenses of vigorous trees. But the likelihood that populations could transition from endemic to epidemic levels due to increased tree susceptibility from wildfire is constrained by the opposing factors of lower nutritional quality and more competition load in severely-injured trees, and the relatively low incidence of moderately-injured trees. Wildfire may cause some reproductive increases in populations that are already in outbreak mode.

Alimentary niche partitioning in the Galapagos sea lion, Zalophus wollebaeki

Sea lions are generally considered opportunistic feeders. However, studies from different areas suggest their diet consists mostly of four to five types of prey. Previous studies in Galapagos sea lions have identified at least three feeding strategies for this species, suggesting diversification of their diet. Diet diversification is favored in organisms with relatively high trophic position and subject to high intra-specific and low inter-specific competition. Zalophus wollebaeki meet these criteria as the only pinniped on San Cristobal Island, where three sea lion rookeries are located within 11 km: a distance considerably shorter than their 41 km foraging range. To measure the degree of diet diversification, we used scats and stable isotope analyses. A total of 270 scat samples from lactating females and 142 fur samples from sea lion pups were collected during the breeding season 2006. The scat analysis identified distinct diets among rookeries, with minimal trophic overlap (Cλ = 0.19), a trophic level TL = 4.5 (secondary–tertiary carnivore), and trophic breadth of a specialist predator (Bi = 0.37). The mean δ15N and δ13C values were 13.07 ± 0.52 and −16.34 ± 0.37, respectively. No significant difference was found in the δ15N values from the sea lion rookeries, but differences were found inter- and intra-population in δ13C values for pups from different groups (ANOVA P < 0.05). Our results indicate that diet diversification is present in the Galapagos sea lion and may play important role to the survival of the species in a habitat where pinniped populations are limited.

Stabilizing selection on genome size in a population of Festuca pallens under conditions of intensive intraspecific competition

*Stabilizing selection is a key evolutionary mechanism for which there is relatively little experimental evidence. To date, stabilizing selection has never been observed at the whole-genome level. *We tested the effect of selection on genome size in a field experiment using seeds collected in a population of Festuca pallens with a highly variable genome size. Using flow cytometry, we measured the genome size in germinating seedlings and juvenile plants grown with or without high intraspecific competition (908 individuals). Above-ground biomass and leaf number were used as measurements of individual vegetative performance. The possible confounding effect of seed weight was controlled for in a separate experiment. *Growth under high competition had a significant stabilizing effect on genome size. Because no relationship was observed between genome size and vegetative performance, we assume that the elimination of plants with extreme genome sizes was the result of decreased survival as a consequence of some unrecognized stress. *Our results indicate that genome size may be under direct selection. The equal disadvantaging of either large or small genomes indicates that the selection for optimum genome size in species may be fully context dependent. This study demonstrates the power of competition experiments for the detection of weak selection processes.

Effects of nest size and dispersion on brood production in a North American population of wood ant Formica fusca (Hymenoptera: Formicidae)

AbstractWe examined several key parameters of the population ecology of a North American population of Formica fusca (L.), including nest dispersion, colony size and brood production. Physical nest size was significantly correlated with colony size, and colony size, in turn, was significantly correlated with brood production. Sex allocation was male biased, although larger nests were more likely to produce reproductive female brood (gynes). Neither nest temperature nor moisture level was significantly correlated with brood production. Formica fusca nests in this population had a comparatively low average nearest‐neighbor distance with a significantly even pattern of dispersion, which suggests relatively high intraspecific competition. However, nearest‐neighbor distance was not significantly associated with either colony size or relative brood production.

Intraspecific competition delays recovery of population structure

Ecotoxicological field studies have shown that total abundance and biomass often recover shortly after pulsed toxicant stress. In contrast, population structure showed comparatively long-term alterations before reaching pre-treatment conditions. We investigated two mechanisms that may explain the prolonged recovery of population structure: latent toxicant effects on life-history traits on the individual level and competition on the population level. To test these hypotheses we exposed populations of Daphnia magna to a pulse of the pyrethroid Fenvalerate. For several generations the populations were kept at two different degrees of competition: strong competition at carrying capacity and reduced competition maintained by simulated predation. After disturbance due to Fenvalerate exposure, biomass recovered after 14–17 days. In contrast, size structure characterised by a lack of large and dominance of small organisms recovered after 43 days in populations with strong competition. Size structure recovered twice faster in populations with reduced competition. We explain this as follows: due to toxicant induced mortality, food availability and consequently birth rate increased and populations were dominated by small individuals. In populations without predation, these cohorts grew and eventually exerted high intraspecific competition that (i) stopped further growth of juveniles and (ii) increased mortality of adults. These demographic processes were mainly responsible for the prolonged recovery of size structure. In contrast, for populations with predation, the regular harvest of individuals reduced competition. Juveniles developed continuously, allowing a fast recovery of size structure in these dynamic populations. In risk assessment the duration for populations to recover from (toxicant) stress, is crucial for the determination of ecological acceptable effects. We conclude that competition needs to be considered in order to understand and predict recovery of size structure.

Behavioral correlations provide a mechanism for explaining high invader densities and increased impacts on native prey

The fact that superabundant invasive pests are also sometimes highly aggressive represents an interesting paradox. Strong intraspecific aggression should result in high intraspecific competition and limit the densities reached by exotic species. One mechanism that can allow invaders to attain high densities despite high intraspecific aggression, involves positive correlations between aggression and other behaviors such as foraging activity. We conducted a mesocosm experiment to quantify the ecological implications of correlations between aggressiveness and foraging activity among groups of exotic signal crayfish (Pacifastacus leniusculus) at low and high densities. Our results showed that high invader densities increased intraspecific aggression and per capita interactions between crayfish, but also increased foraging activity and impacts on preferred prey. As a result, exotic crayfish did not show density-dependent reductions in per capita feeding or growth rates. We suggest that the positive correlation between aggression and activity is part of an aggression syndrome whereby some individuals are generally more aggressive/active than others across situations. An aggression syndrome can couple aggressive behaviors important to population establishment of invasive species with foraging activity that enhances the ability of invaders to attain high densities and have large impacts on invaded communities.

Egg size affects larval performance in a coleopteran parasitoid

Abstract 1. Optimal progeny size models assume that the more eggs a female produces, the lower the amount of resource allocated per egg. As egg size generally correlates with the fitness of the emerging immature, this trade‐off can be expressed as a choice between the production of numerous low quality or fewer high quality progeny.2. The first‐instar larvae of the coleopteran parasitoid Aleochara bilineata have to search for and parasitise dipteran pupae. The present study found a positive correlation between egg size and larval weight, but not between egg size and development time. Larger first‐instar larvae survived longer, were more active, and found and parasitised their host more rapidly.3. Female A. bilineata may invest smaller larvae in conditions of high host density and low intraspecific competition, but investing fewer, larger larvae would bring them more fitness when hosts are scarce and competition high.

Island selection on mammalian life-histories: genetic differentiation in offspring size

BackgroundSince Darwin's pioneering work, evolutionary changes in isolated island populations of vertebrates have continued to provide the strongest evidence for the theory of natural selection. Besides macro-evolutionary changes, micro-evolutionary changes and the relative importance of natural selection vs. genetic drift are under intense investigation. Our study focuses on the genetic differentiation in morphological and life-history traits in insular populations of a small mammal the bank vole Myodes glareolus.ResultsOur results do not support the earlier findings for larger adult size or lower reproductive effort in insular populations of small mammals. However, the individuals living on islands produced larger offspring than individuals living on the mainland. Genetic differentiation in offspring size was further confirmed by the analyses of quantitative genetics in lab. In insular populations, genetic differentiation in offspring size simultaneously decreases the additive genetic variation (VA) for that trait. Furthermore, our analyses of differentiation in neutral marker loci (Fst) indicate that VA is less than expected on the basis of genetic drift alone, and thus, a lower VA in insular populations could be caused by natural selection.ConclusionWe believe that different selection pressures (e.g. higher intraspecific competition) in an insular environment might favour larger offspring size in small mammals. Island selection for larger offspring could be the preliminary mechanism in a process which could eventually lead to a smaller litter size and lower reproductive effort frequently found in insular vertebrates.

Maternal care in white-tailed deer: trade-off between maintenance and reproduction under food restriction

Lactation is the most energetically costly component of maternal investment in mammals. For large temperate herbivores, summer is characterized by relatively abundant forage but also high energetic needs for lactation and recovery from winter mass loss. We experimentally restricted food supply by about 20%, and compared the nursing and foraging behaviours of control and food-restricted adult female white-tailed deer, Odocoileus virginianus, and their fawns during lactation. We considered two fawn ages: 0–30 days (after which time spent suckling dropped markedly) and 30–80 days (the end of the nursing period). From 0 to 30 days of age, food-restricted fawns performed 17 more suckling bouts/day and spent twice as much time suckling than control fawns. Compared with controls, food-restricted fawns gained 26% less mass from birth to 80 days. Body growth was inversely related to time spent suckling and to the frequency of nursing bouts, but positively related to survival. Food-restricted fawns had twice as many suckling solicitations and rejected suckling attempts as control fawns. Solicitations for allosuckling and successful allosuckling bouts were also more than twice as high in the food-restricted group as in the control group. Mothers and fawns from the food-restricted group spent more time foraging than control individuals. We conclude that a reduction in food availability during summer, which may occur under high intraspecific competition, should lead to drastic changes in foraging and nursing behaviours as well as reduced growth rate of juveniles of large northern herbivores.

Demography of the Invasive Geophyte Oxalis pes-caprae Across a Mediterranean Island

Success during the early stages of the life-history of alien plants is essential for invasion to occur. The reproductive components of plant invaders have mostly been studied in species reproducing sexually but little is known about invaders that depend exclusively on vegetative reproduction. In this paper, the importance of the different recruitment stages on population growth is quantified and, thus, the invasion potential of the South African annual geophyte Oxalis pes-caprae invading Mediterranean ecosystems is assessed. Tests and experiments were conducted across Menorca (Balearic Islands) to analyse the spatial variability of Oxalis pes-caprae reproductive components (i.e. bulb production, bulb bank, bulb predation, bulb mortality, bulb dormancy, bulb germination, plant establishment and survival). Oxalis pes-caprae has a transient bulb bank that remains dormant in the soil during summer. High levels of bulb predation after dispersal, followed by bulb mortality during summer or a failure to germinate in autumn were the most critical factors limiting plant establishment. Bulb germination was high. However, plant establishment and bulb production is constrained by intraspecific competition, but is not affected by soil disturbance. No symptoms of spatial discordance could be found between recruitment stages because the spatial variability of the life cycle was extremely low at all the scales examined (i.e. among populations, habitats and microsites). It was estimated that, on average, 4 % of bulbs can become plants the following year and the field rate of population increase (lambda) to be 0.08. The results suggest that invasion is constrained by post-dispersal bulb predation, loss of viability of the propagule bank due to summer drought and high intraspecific competition. However, a high spatial concordance between recruitment stages and probably a high propagule pressure due to human and livestock bulb dispersal determine the success of this invader across Menorca Island.

Effects of competition on an endangered limpet Patella ferruginea (Gastropoda: Patellidae): Implications for conservation

Spatial and trophic competitive interactions were analysed for the seldom-studied limpet Patella ferruginea (which is considered as an “extinction risk”). Spatial patterns of distribution of P. ferruginea were studied using quadrats of 1 × 1 m 2 at several transects along the coastline, whereas trophic interactions were studied through manipulations of densities of P. ferruginea and Patella caerulea in experimental plots located at the mid-shore. Physical parameters were measured inside each plot to determine the microenvironmental differences between fenced and unfenced plots. Fenced plots of 50 × 50 cm 2 were used with six different treatments. Mortality was recorded every 15 days and length was measured monthly during the 3.5 months of the experiment. At the beginning and end of the experiment, lengths and weights of limpets were recorded and chlorophyll concentration was measured by spectrophotometric analysis of rock surface. Both species showed increased mortality and reduced size and weight in treatments with greater densities. High intraspecific competition was detected in P. caerulea compared with P. ferruginea whereas asymmetrical interspecific competition was observed. The results suggest that P. caerulea is the superior competitor, possibly due to differences in radular morphology and mobility between the two species. Microalgal food assessment showed that it is probably a limiting resource, especially for P. caerulea, although this species has a lower biomass than P. ferruginea. The high intraspecific competition observed in P. caerulea could be responsible for the coexistence of both species. The present study may be useful in helping to conserve P. ferruginea, which is presently under serious threat of extinction, because the results show that experimental fences are a viable means of concentrating breeding individuals.

Intra-specific competition in maize: Contribution of extreme plant hierarchies to grain yield, grain yield components and kernel composition

Maize ( Zea mays L.) cropping conditions that promote high intra-specific competition pressure generate an increased plant-to-plant variability within the stand, and the appearance of individuals with different ability to capture scarce resources (i.e. dominant and dominated plants). The objectives of this paper were to analyze (i) stand density effects on plant biomass at physiological maturity (R 6), grain yield per plant (GYP), GYP components (KNP: kernel number per plant; KW: kernel weight), and kernel composition (starch, oil and protein contents per kernel) of the mean plant of the stand (i.e. considering all individuals) and of the dominant (D) and dominated (d) individuals; and (ii) the contribution of these extreme plant hierarchies to GYP, GYP components and kernel composition of the mean plant of a stand. Four maize hybrids of contrasting KW (small and large KW) were cropped at a wide range of stand densities (3–15 pl m −2) during 1999/2000 and 2001/2002 in Argentina. The mean value of measured variables declined as plant density increased from 3 to 15 pl m −2, and plant-to-plant variability (CV: coefficient of variation) of the same variables increased with enhanced crowding. The magnitude of the reduction in mean plant values differed among variables: plant biomass at R 6, GYP and KNP underwent a larger reduction (ca. 66%) than KW (ca. 14–19%) or kernel contents (ca. 22% for oil and protein contents, and 13% for starch content). Similarly, the increase in CVs was larger for plant biomass at R 6 (from ca. 13 to 40%) and GYP (from ca. 30 to 58 and 15 to 38% for small and large KW hybrids, respectively) than for KW (ca. from 7 to 20%). Only a slight increase in CVs of oil (6–17%) and protein (9–12%) concentrations of large KW hybrids was recorded. The CV of KNP followed a trend similar to that for GYP. Differences between plant categories increased when mean GYP and KNP of all individuals of the stand were smaller than 157 g pl −1 and 649 kernel pl −1, respectively. Below these thresholds, the d/D ratio dropped from 0.76 to 0.30 (small KW hybrids) or to 0.40 (large KW hybrids) for GYP ( r 2 = 0.76, P < 0.001), and from 0.75 to 0.38 (small KW hybrids) or to 0.46 (large KW hybrids) for KNP ( r 2 = 0.59, P < 0.001). In contrast, the d/D ratio for KW varied always from 1 to 0.80 in response to decreased mean KW ( r 2 = 0.39, P < 0.01). The concentration of kernel contents did not differ between plant types. Results indicate that grain yield of maize crops grown at high stand densities is composed by plants bearing very different kernel numbers, with slight differences in kernel size, and similar starch, oil and protein concentration.

Size and shape changes of Ips typographus L. (Coleoptera: Scolytinae) in relation to population level

Abstract1 High intraspecific competition is known to occur during Ips typographus outbreaks, and is thought to be the main factor regulating epidemic populations by affecting beetle population productivity. However, little is known about the consequences of intraspecific competition on population quality during outbreaks, although it could have consequences on beetle population dynamics.2 Ips typographus morphological variations among localities, years and beetle population levels were investigated in 10 Norway spruce (Picea abies) stands having various beetle damage intensities. Beetle size and shape estimators based on wing length, and using isometric size and log‐shape ratios, were employed. Field‐caught beetles were compared with beetles emerging from controlled breeding at different densities, performed in the laboratory. Beetles from this colony were also used to check the influence of breeding densities on the size estimator.3 Size variations occurred among localities and years and were consistent with the epidemic or latent status of the beetle populations. Controlled breeding confirmed the negative effect of beetle densities encountered in the field on offspring size. Two hypotheses are formulated to explain this increase of intraspecific competition during an outbreak, but our data support the effect of host quality change between latent and epidemic populations.4 Shape variations also occurred among localities but were unrelated to beetle population levels. No groups consistent with a geographical structure were found, suggesting low genetic variation for I. typographus populations in France.

Growth, productivity, and competitiveness of introgressed weedy Brassica rapa hybrids selected for the presence of Bt cry1Ac and gfp transgenes

Concerns exist that transgenic crop x weed hybrid populations will be more vigorous and competitive with crops compared with the parental weed species. Hydroponic, glasshouse, and field experiments were performed to evaluate the effects of introgression of Bacillus thuringiensis (Bt) cry1Ac and green fluorescent protein (GFP) transgenes on hybrid productivity and competitiveness in four experimental Brassica rapa x transgenic Brassica napus hybrid generations (F1, BC1F1, BC2F1 and BC2F2). The average vegetative growth and nitrogen (N) use efficiency of transgenic hybrid generations grown under high N hydroponic conditions were lower than that of the weed parent (Brassica rapa, AA, 2n = 20), but similar to the transgenic crop parent, oilseed rape (Brassica napus, AACC, 2n = 38). No generational differences were detected under low N conditions. In two noncompetitive glasshouse experiments, both transgenic and nontransgenic BC2F2 hybrids had on average less vegetative growth and seed production than B. rapa. In two high intraspecific competition field experiments with varied herbivore pressure, BC2F2 hybrids produced less vegetative dry weight than B. rapa. The competitive ability of transgenic and nontransgenic BC2F2 hybrids against a neighbouring crop species were quantified in competition experiments that assayed wheat (Triticum aestivum) yield reductions under agronomic field conditions. The hybrids were the least competitive with wheat compared with parental Brassica competitors, although differences between transgenic and nontransgenic hybrids varied with location. Hybridization, with or without transgene introgression, resulted in less productive and competitive populations.